The present invention relates generally to medical devices and particularly to a medical implant or stent made from natural tissues.
The use of tubular medical implants to treat various organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like, has become common. Typically, tubular medical implants (e.g., stents, prosthesis, grafts and other such devices) are useful in treating blockages, occlusions, narrowing ailments and other similar problems that restrict flow through a passageway.
One such medical treatment involves the use of an endovascular stent that is implanted in the vascular system. Stents are useful for numerous medical treatments of various vessels throughout the vascular system, including both coronary vessels and peripheral vessels (e.g., carotid, brachial, renal, iliac and femoral). However, the use of stents in coronary vessels has drawn particular attention from the medical community due to the commonality of heart problems caused by stenosis.
Although stenosis (i.e., narrowing of a vessel) may occur for a variety of reasons, one of the most common causes of coronary stenosis results from the buildup of atherosclerotic plaques along the lumen of the vessel. The resulting coronary stenosis restricts blood flow through the vessel, which eventually can lead to a dangerously increased risk of heart attacks.
The medical community has attempted to address coronary stenosis (along with the many other passageway problems that patients suffer from) with various versions of percutaneous transluminal angioplasty (“PTA”). Fundamentally, PTA involves inserting a balloon-tipped catheter into a vessel and threading the catheter to the narrowed portion to be treated. The balloon is then expanded at the narrowed portion by pumping saline through the catheter to the balloon. As a result, the balloon expands, contacts the inner vessel wall, and forces the vessel to dilate. The balloon is then deflated and retracted from the vessel.
One problem that has been encountered with the described version of PTA is restenosis (i.e., a re-narrowing) of the vessel. Restenosis may occur for a variety of reasons, such as collapsing of the vessel wall or regrowth of cellular tissue. For example, restenosis is frequently caused by damage to the vessel lining which occurs during balloon expansion and vessel dilation. As a result of the damage caused to the intima layers of the vessel, the vessel attempts to grow new intima tissue to repair the damage. This tendency of vessels to regrow new tissues is referred to as neointimal hyperplasia. The effect of this response results in a re-narrowing of the vessel. Restenosis however is not completely predictable and may occur either abruptly soon after the PTA procedure or may occur slowly over a longer period of time.
One approach the medical community has tried to overcome the problems with restenosis is to use stents in conjunction with the above-described PTA procedure. Traditionally, stents are made of metal or other synthetic materials, thereby providing a tubular support structure that radially supports the inner wall of the vessel. The most common materials now used in stents are stainless steel (e.g., 316L SS and 304 SS) and Nitinol. Typically, stents are designed with a plurality of openings extending through the support structure in a manner that permits the stent to radially expand from a small diameter to a larger diameter. Thus, when used in conjunction with conventional PTA procedures, the stent is positioned within the portion of the vessel that has been widened by the balloon and is permanently implanted by radially expanding the stent against the inner wall of the vessel. The expectation of this revised PTA procedure is that the support structure of the implanted stent will mechanically prevent the vessel from collapsing back to its original narrowed condition.
Restenosis however can still be a problem even when a stent is used in conjunction with PTA procedures. As discussed above, one problem is neointimal hyperplasia caused by damage to the vessel wall. This can also be a problem when a stent is used. In addition, neointimal hyperplasia can also be caused by the synthetic materials that are usually used in stents. The reason for this problem is that living tissues have a tendency to grow new living tissues around and over foreign objects that are implanted into the body. Thus, despite the mechanical support structure provided by a stent, restenosis remains a problem. One approach that has been offered to address these problems is coating the stent with drugs that are designed to inhibit cellular regrowth. Common examples of such drugs include Paclitaxel, Sirolimus and Everolimus. One problem with conventional drug coatings, however, is that the drug coating is only available at the outer surface of the stent and is quickly released after implantation. Thus, the drug is only effective for a short period of time.
Although stent designs and implantation procedures vary widely, two categories are common.
The first of these two categories may be referred to as balloon-expanding stents. Balloon-expanding stents are generally made from soft ductile materials that plastically deform relatively easily. In the case of stents made from metal, 316L stainless steel which has been annealed is a common choice for these types of stents. One common procedure for implanting a balloon-expanding stent involves mounting the stent circumferentially on the balloon prior to threading the balloon-tipped catheter to the narrowed vessel portion that is to be treated. When the balloon is positioned at the narrowed vessel portion and expanded, the balloon simultaneously dilates the vessel and also radially expands the stent into the dilated portion. The balloon and the catheter are then retracted, leaving the expanded stent permanently implanted at the desired location. Ductile metal lends itself to this type of stent since the stent may be compressed by plastic deformation into a small diameter when mounted onto the balloon. When the balloon is then expanded in the vessel, the stent is once again plastically deformed into a larger diameter to provide the desired radial support structure. Traditionally, balloon-expanding stents have been more commonly used in coronary vessels than in peripheral vessels due to the deformable nature of these stents. One reason for this is that peripheral vessels tend to experience frequent traumas from external sources (e.g., impacts to a person's arms, legs, etc.) which are transmitted through the body's tissues to the vessel. In the case of peripheral vessels, there is an increased risk that an external trauma could cause a balloon-expanding stent to once again plastically deform in unexpected ways with potentially severe and/or catastrophic results. In the case of coronary vessels, however, this risk is minimal since coronary vessels rarely experience traumas transmitted from external sources.
A second common category of stents is referred to as self-expanding stents. Self-expanding stents are generally made of shape memory materials that act like a spring. Typical metals used in these types of stents include Nitinol and 304 stainless steel. A common procedure for implanting a self-expanding stent involves a two-step process. First, the narrowed vessel is dilated with the balloon as described above. Second, the stent is implanted into the dilated vessel portion. To accomplish the stent implantation, the stent is installed on the end of a catheter in a compressed, small diameter state and is retained in the small diameter by inserting the stent into the lumen of the catheter or by other means. The stent is then guided to the balloon-dilated portion and is released from the catheter and allowed to radially spring outward to an expanded diameter until the stent contacts and presses against the vessel wall. Traditionally, self-expanding stents have been more commonly used in peripheral vessels than in coronary vessels due to the shape memory characteristic of the metals used in these stents. One advantage of self-expanding stents for peripheral vessels is that traumas from external sources do not permanently deform the stent. Instead, the stent may temporarily deform during an unusually harsh trauma but will spring back to its expanded state once the trauma is relieved. Self-expanding stents, however, are often considered to be less preferred for coronary vessels as compared to balloon-expanding stents. One reason for this is that balloon-expanding stents can be precisely sized to a particular vessel diameter and shape since the ductile metal that is used can be plastically deformed to a desired size and shape. In contrast, self-expanding stents are designed with a particular expansible range. Thus, after being installed self-expanding stents continue to exert pressure against the vessel wall.
One problem with traditional stents is the metallic and other synthetic materials that are used to make the stents. As mentioned above, the most common material now used to make stents is stainless steel and other similar metals. Other synthetic materials which are sometimes used in stents include various types of polymers. It is well known that the human body is generally resistant to the implantation of foreign materials into the body. For example, in the case of stainless steel (which is used in most stents), it is known that a certain percentage of people are allergic to the nickel contained in stainless steel. Because of the known, general risk of implanting synthetic materials into the human body, the medical community must extensively test any new material or new application before a medical device may be considered safe for permanent implantation. Even then, the response that a particular human body may exhibit to a particular synthetic material can be unpredictable.
Some attempts to address this concern with implanting foreign materials into the human body have relied upon stents made either partly or completely from bioabsorbable/biodegradable materials. Essentially, these materials are polymers that breakdown over time until the original implanted material is either partially or wholly dispersed into the body. Some examples of these types of materials include poly(L-lactic acid), poly(glycolic acid), polycaprolactone and various copolymers thereof. These materials however do not adequately solve the problems with traditional stents and may even cause new problems. For example, these stents do not fully address the concern with implanting synthetic materials into the body, since the bioabsorbable/biodegradable materials that are used are themselves synthetic with potentially unpredictable physiological responses thereto. Moreover, these stents are specifically designed to disintegrate over time into smaller pieces. This may result in a possible embolism if a larger piece unexpectedly breaks off from the stent and passes through the vascular system. Moreover, even when the polymer material degrades into small pieces that avoid the risk of embolisms, the breakup of the polymer material still results in non-natural, synthetic materials being dispersed in an uncontrolled manner throughout the body.
Accordingly, a solution that avoids these and other problems is described more fully below.